Canadian hemlock is a fairly fast growing evergreen that will fill in quickly along your border. The only problem is these are trees that are grow to 50 feet or more tall. You can yearly trim them to keep the height within the 12 feet tall you desire, but a better evergreen choice may be the white or red cedar. They don't grow as large, will fill in nicely over time and although they too need year trimming, are a bit more manageable as a hedge than the hemlock. Check your local nursery for stock.